Long-life, high-reliability, space-qualified cryocoolers are a major enabling technology for infrared sensor systems on satellites. A linear drive, non-contacting bearing Stirling cycle cryocooler concept is the best approach. (Johnson, A. L., "Spacecraft Borne Long Life Cryogenic Refrigeration Status and Trends", Cryogenics, pp. 339-347, July 1983, and, Henderson, B. W., "U.S. Industry Close to Producing Long Life Space Cooling System", Aviation Week & Space Technology, 6 Apr. 1992). The flexure bearings provide the frictionless and non-wearing support for the reciprocating components of the cryocooler. The high radial stiffness of these bearings allows the use of non-contacting gas gap clearance seals. These bearings were first applied to cryocoolers by Oxford University with spiral-cut diaphragms (Davey, C., "The Oxford University Miniature Cryogenic Refrigerator", International Conference on Advanced Infrared Detectors and Systems, London, pp. 39, 1981). The use of these bearings in an artificial heart application has been disclosed, (Johnston, R. P., et. al., "A Stirling Engine With Hydraulic Power Output For Powering Artificial Hearts", Paper 75212, IECEC Record 1975). Another use of this bearing is in an ultraviolet sensor shutter system, (Curtis, P. D., et al., "Remote Sounding of Atmospheric Temperature From Satellites and V--The Pressure Modulator Radiometer for Nimbus F" , Proc. R. Soc. London, A 337, pp. 135-150, 1974).
Although the above bearings have provided one approach to the reciprocating machines such as the cryocooler, a need exists for a high radial to axial stiffness ratio, low operating stress, long life, high reliability, and space-qualified flexure bearing. Spiral flexure spring response characteristics have been investigated as a suitable means. In general, the spiral flexure bearing assembly consists of a stack of axially flexible cut spiral diaphragms deposed between inner and outer hub spacers and rim spacers which are fastened to a compressor housing which provides support to the flexure spiral diaphragm. An ideal flexure bearing should have the characteristics of a very large radial or in-plane stiffness, minimal axial or out-of-plane stiffness, and minimal stresses when deflected. The radial stiffness is needed in the reciprocating machine to maintain the extremely tight clearance between the piston and the cylinder such as in a compressor to form a gas clearance seal. The axial stiffness needs to be kept as low as possible to avoid affecting the natural frequency of the spring mass system composed of the piston and compressible gas. Low stresses in the cut diaphragm are particularly required to assure that the bearing will not fail due to fatigue stress. Finite element analysis of the spiral-cut diaphragm flexure indicated that highly concentrated stresses occurred at very small regions in the spiral legs near the rim and the hub. These stresses are caused by a combination of bending, tensile, and in particular, undesirable torsional warping. To assure a long life and high fatigue reliability of the flexure bearing over 10,000,000,000 cycles, the magnitude of these undesirable stresses has to be minimized.
One of the possible approaches would be to more effectively utilize the flexure material and distribute the high stresses over a large region of the flexure. Another approach would be to eliminate the geometric stress concentration by pushing the peak stress away from the spiral leg hub and rim. The third approach is to minimize the flexure stress by reducing the undesirable torsional warping of the spiral-cut diaphragm flexure during operation.
A superior solution is a tangential linear flexure bearing design utilizing a translating spider diaphragm having three circumferential tangential cantilever flexure blades. These cantilever flexure blades are sandwiched, riveted, brazed or otherwise restrained between the hub and rim spacers. The flexure support points, of the tangential cantilever blades are defined by the shape of the hub and rim spacers. This flexure design reduces all of the shortfalls identified in the spiral-cut flexures.
Under small axial deflections, the tangential cantilever flexure blades behave like a simple beam under pure bending. Under large axial deflection, as in the case of cryocooler applications, the amount of undesirable warping of the flexure blades is significantly reduced when compared to that of an equivalent spiral cut flexure. Therefore, the deflected flexure blades tend to stress primarily in bending. The combined stress is well behaved and almost uniformly distributed across the width of the flexure blade at full stroke. Peak stresses also tend to be widely distributed and significantly less concentrated than those of the spiral-cut diaphragm. The radial to axial stiffness ratio exceeds 1000.
Stress analysis results for an unimproved tangential linear flexure bearing design has been documented, (Wong, T. E., et. al., "Novel Linear Flexure Bearing" presentation at the 7th International Cryocooler Conference on Nov. 18, 1992, and at the ASME 16th Energy-sources Technology Conference & Exhibition on Feb. 1, 1993). The previously disclosed tangential linear flexure bearing has trapezoidal off set flexures with bonds at the ends joining the hub and rim. The offset of the tangential flexure blades relative to a line extending radially from the center of the diaphragm creates increased maximum shear stress points along length of the flexures. The flexure blades also have apparent random grain orientations along the flexure blades further increasing maximum allowable stress along the length of the flexures. It has been discovered by the inventors of the present invention that perpendicular flexure bonds at the rim and hub attachment points do not distribute stress as evenly along the length of the flexure blades and random grain orientations limits the allowable stress. These and other disadvantages are reduced using an improved tangential linear flexure bearing design according to the teachings of the present invention.